As shown in FIG. 5, a known flat type IC package is in the shape of flat rectangular parallelpiped. Lead terminals 2 are aligned at the edge. In FIG. 5, these lead terminals project like legs of a centipede, but they may instead by round shaped loops.
A known IC socket 4 for testing said flat type IC package 1, as shown in FIGS. 5, 6, has a socket body 6 having a depression 5 for inserting the IC package 1, contact pins 3 planted on the socket body 6 and a cover 7 pivoted on the socket body 6 for pressing the IC package 1 or lead terminals 2 so as to contact the lead termianals 2 to contact pins 3 respectively. Cover 7 is locked in a closed position a lock lever 8 which is provided on the socket body 6 as shown in FIG. 5 or the cover 7 as shown in FIG. 6.
Said contact pins 3 are formed by stamping a thin resilient sheet metal plate, which are provided in the socket body 6 so as to correspond to said lead terminals 2. It is usual to form the upper part of the contact pin 3 to a hairpin like shape for resilient contact with lead terminal 2.
However, this shape of the contact pin 3 makes a loop circuit and it creates a longer circuit and high self-inductance circuit in the order of 10 nH. This high self-inductance hinders accurate measuring of high frequency characteristics, especially in the high frequency range above 100 MHz. U.S. Pat. No. 4,354,729 proposes a contact pin which forms a linear circuit, but it damaoes the lead terminal of the IC package due to slipping between them, and needs a step for preloading the spring arm in manufacturing.